The present invention relates to a magnetic head employing a magnetoresistive sensor and a magnetic storage and retrieval system.
Increase in recording density is essential to magnetic storage and retrieval apparatuses, particularly, magnetic disk apparatuses to meet both a progressively increasing demand for capacity increase and a demand for apparatus miniaturization. There are many problems to be solved to achieve increase in recording density. The reproducing head for reading signals magnetically recorded on a recording medium and converting the signals into electric signals is a key device of a magnetic disk drive(apparatus). Miniaturization according to recording density and sensitivity enhancement are essential requisites for the reproducing head.
Recently, MR heads employing a magnetoresistive sensor are used as a reproducing head capable of meeting those two essential requisites. The MR head is capable of producing signals at an S/N ratio (signal-to-noise ratio) higher than that at which the conventional inductive head produces signals. The magnetoresistance effect currently used by the MR head is called an anisotropic magnetoresistance effect (AMR effect). The AMR effect is the dependence of the resistance of a magnetic body when a current flows through the magnetic body on the angle between the direction of magnetization and the direction of the current. It is known that the AMR effect is capable of causing resistance changes in the range of about 2% to about 5%. Recording density which can be achieved by the MR head using the AMR effect is several gigabits per square inch at the highest. The magnetoresistance effect, which is more sensitive than the AMR effect, must be used to achieve recording at a higher recording density. The giant magnetoresistance effect (GMR effect) is a prospective means for achieving recording at a higher recording density.
The GMR effect is the variation of the electrical resistance of a multilayer film formed by alternately laminating magnetic metallic films and nonmagnetic metallic films according to angles between the directions of magnetization of the magnetic metallic films. The GMR effect achieves a large resistance change ratio exceeding 5% at a room temperature. The most prospective multilayer film for application to a magnetic head is a spin valve film. The spin valve film consists substantially of four layers. The spin valve film comprises two ferromagnetic layers and a nonmagnetic layer sandwiched between the ferromagnetic layers. An antiferromagnetic film is laminated to one of the ferromagnetic layers. Generally, the ferromagnetic layer combined with the antiferromagnetic layer is called a fixed layer and the other ferromagnetic layer is called a free layer. The direction of magnetization of the fixed layer is fixed by the exchange interaction of the fixed layer and the antiferromagnetic layer so that the angle between the direction of magnetization of the free layer and that of the fixed layer is 90.degree.. In view of the application of the spin valve film to a magnetic head, it is desirable that the free layer have a direction of magnetization along track width and the fixed layer has a direction of magnetization along the height of elements. The basic construction of a head employing the spin valve film is disclosed in, for example, JP-A No. Hei 4-358310.
It is known that device errors which have not been experienced by a magnetic disk drive provided with an inductive head arise when an MR head is applied to a magnetic disk drive(apparatus). The most significant one of the device errors is the vertical asymmetry of reproduced waveforms attributable to the dependence of the magnitude of output pulses corresponding to a magnetic field created by the recording magnetization of the medium on the polarity of the magnetic field.
In the spin valve MR head, when the direction of magnetization of the fixed layer is fixed firmly in the direction of element height (direction along a normal to the recording medium), and the direction of magnetization of the free layer is ideally free to turn about the longitudinal direction of element (direction along the width of tracks), the vertical asymmetry will not occur. The direction of magnetization of the fixed layer is firmly pinned down (fixed) in the direction of element height by a unidirectional anisotropy based on the exchange interaction between the ferromagnetic layer and the antiferromagnetic layer to achieve such an ideal arrangement of the directions of magnetization. However, if the direction of magnetization of the fixed layer is pinned down infirmly and the direction of magnetization of the fixed layer is unstable in an external magnetic field, part of the magnetic flux generated by the magnetization of the recording medium leaks into the fixed layer and turns the direction of magnetization of the fixed layer. Consequently, the magnetic flux that flows into the free layer is reduced and the dynamic range of rotation of the direction of magnetization of the free layer is limited. In such a case, since the magnetic flux that flows into the fixed layer is dependent on the direction of magnetic flux, the amount of rotation of the direction of magnetization of the free layer varies according to the direction of the magnetic field created by the recording medium and, consequently, the vertical asymmetry of playback signals is enhanced.
Although the free layer is directed substantially in the longitudinal direction by the intrinsic uniaxial anisotropy of the ferromagnetic layer, the uniformity of magnetization distribution of the free layer is disturbed by a static magnetic field created by the magnetization of the fixed layer and hence it is possible that the linearity of playback signals is deteriorated.
JP-A No. Hei 7-169026 proposes, to improve the distribution of magnetization in the free layer, a laminated film having a fixed layer consisting of a ferromagnetic film, a nonmagnetic metal film and a ferromagnetic film laminated in that order. In the fixed layer, the two ferromagnetic films are coupled by antiferromagnetic coupling through the nonmagnetic metal film to magnetize the two ferromagnetic films so that their magnetic moments are aligned in an antiparallel fashion. The direction of magnetization of one of the ferromagnetic films of the fixed layer is fixed by the exchange coupling of the same ferromagnetic film and the antiferromagnetic film contiguous with the fixed layer. Since the magnetic moment of the ferromagnetic film is nullified, the substantial influence of the static magnetic field of the fixed layer on the free layer is negated.
JP-A No. Hei 8-7235 discloses a construction having a laminated fixed layer consisting of a ferromagnetic film, a nonmagnetic metal film and a ferromagnetic film laminated in that order similar to the fixed layer disclosed in JP-A No. Hei 7-169026, and not employing any antiferromagnetic layer. In this construction, the directions of magnetization of the two ferromagnetic films can be fixed only by the uniaxial magnetic anisotropy of the ferromagnetic films.